A thin gold-alloy wire has been mainly used as a bonding wire for bonding electrodes on a semiconductor device to external leads. An ultrasonic wave-thermal pressing combination system has been employed generally as bonding technology of the thin gold-alloy wire. Namely, the distal end of the thin gold-alloy wire is heated and molten by arc heat input to form a ball by a surface tension. After this ball is press-bonded to an electrode of a semiconductor device heated to a temperature within the range of 150.degree. to 300.degree. C., it is connected to an external lead by ultrasonic pressing.
To use semiconductor devices such as transistors, ICs, etc., as the product, a Si chip, bonding wires, and the portions of a lead frame to which the Si chip is fitted, are resin-molded for protection after bonding the thin gold-alloy wire as described above.
The characteristics which the thin gold-alloy wire is required to possess have become diversified with the higher integration density of semiconductor devices and the reduction of their thickness. To cope with high density wiring and narrow pitches, for example, the thin gold-alloy wire must be thin or must be capable of high looping, and in order to reduce the thickness of the semiconductor device, the thin gold-alloy wire must be capable of low looping. To satisfy elongation of the wire, the reduction of the wire diameter and adjustment of the loop height described above, thin gold-alloy wires containing several kinds of elements as the additional elements have been developed, as described, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 61-296731 and 61-172343.
Recently, the environmental condition for the use of semiconductor devices has become more severe. For example, semiconductor devices used inside the engine compartment of an automobile are exposed to high temperatures, e.g., a temperature range of 150.degree. to 200.degree. C., or high humidity. The heat generated at the time of use of semiconductor devices due to their high density packaging cannot be neglected. When a thin gold-alloy wire is used, the drop of long term reliability of the bond with an aluminum electrode has become a problem.
In semiconductor devices used under an environmental condition where heat-resistance is a requisite, a thin aluminum-alloy wire has been used in the past as a bonding wire, and a ceramic package semiconductor device has been utilized. The thin aluminum-alloy wire has the advantage that high reliability can be obtained at the bond with an electrode on a semiconductor device due to bonding the same kind of metal. However, the ceramic package is more expensive than a resin molding. In addition, it is difficult to form a normal ball in air by the use of the thin aluminum-alloy wire. Accordingly, a wedge bonding method has generally been employed as the bonding method, and producibility drops in comparison with the thin gold-alloy wire.
Therefore, the use of the thin aluminum-alloy wire has been limited to specific semiconductors from the aspects of the production cost, producibility, and so forth, and the bonding system using the thin gold-alloy wire, which has an excellent operation factor, a high producibility and a high workability, is expected to be predominate in future.
The problem of the drop of long term reliability of the bond with the aluminum electrode on the semiconductor device has existed when the conventional thin gold-alloy wire is used at elevated temperature environment, e.g., a temperature range of 50.degree. to 200.degree. C. In other words, aluminum as the electrode material and gold undergo mutual diffusion and form intermetallic compounds and generate voids, so that peel and electrical conduction defects occur at the bond.
In order to control the growth of intermetallic compounds, a technique which adds Mn into a thin gold-alloy wire is described in Japanese Unexamined Patent Publication (Kokai) No. 2-215149.
Under such circumstances, a thin gold-alloy wire having high bonding reliability with the aluminum electrode in a high temperature environment has been required.